Trends of Precipitation Acidification and Determining Factors in China During 2006-2015

Precipitation acidification, often termed as acid rain (pH < 5.6), has been recognized as one of the major environmental issues in China since the 1980s and exerted detrimental effects on terrestrial ecosystems. In this study, we present the first investigation on the evolution of precipitation acidification over China during 2006-2015 based on a long-term nationwide precipitation observation network and chemical transport model (Weather Research and Forest model coupled with Chemistry) simulations. The observations demonstrate a clearly decreasing trend in precipitation acidification across China from 2006 to 2015. The percentages of the number of the stations experiencing heavy acidic precipitation (annual mean pH < 5.0) decrease from similar to 50% in 2006 to 20% in 2015, and those with pH < 4.5 decrease from 25% to less than 5%. Our model simulations reproduced this trend and show the notable decrease in the terrestrial area (from 30% to 16%) experiencing the mean precipitation pH < 5.6. The simulation experiments indicate that the mitigation of the acidification issue in China stemmed from the large reduction in SO2 emissions (by about 60% from 2006 to 2015), which would limit the formation of sulfate and also increase the wet scavenging of gaseous ammonia. The contributions of sulfuric acid on precipitation acidification were estimated to decrease from 0.70-0.82 to 0.44-0.60 from 2006 to 2015 over northern, southern, and southwestern China. This study suggests that nitric acid plays a more important role in acidifying precipitation in northern China at present, while sulfuric acid is still the dominant factor in southern areas.